A psychologist who examined one of the defendants on trial at Guantanamo Bay, Cuba, in connection with the terrorist attacks of Sept. 11, 2001, concluded that the high-value detainee was mentally incompetent, raising questions about his fitness to stand trial, according to individuals familiar with the finding.

For three days in January 2009, Xavier Amador, a professor at Columbia University, interviewed Ramzi Binalshibh, an al-Qaeda operative and protege of alleged Sept. 11 co-conspirator Khalid Sheik Mohammed. He determined that Binalshibh didn’t understand the proceedings against him and suffered from delusions, said one of the individuals who has reviewed the case.

Amador’s assessment, which has not been known publicly until now, comes as the military commission case against Binalshibh, 41, has stalled over questions about his mental health and whether he is fit to stand trial with four other suspected al-Qaeda members, including Mohammed. He was held for years at secret CIA prisons, where he was subjected to “enhanced interrogation techniques” that human rights advocates have described as torture.

The psychologist’s conclusion could presage a similar finding by a medical review board that was ordered by a judge last week to evaluate Binalshibh. Such a determination could cast into doubt the prosecution of one of the key defendants in the Sept. 11 attacks.